This invention relates to correction coils for use in shimming high strength magnets to produce a magnetic field with a highly uniform axial component. More particularly, the present invention relates to an axisymmetric correction coil set disposed on a cylindrical form for use in shimming main magnet coils used in NMR medical imaging diagnostic applications.
Nuclear magnetic resonance (NMR) phenomena, once limited to chemical analysis, are now being employed in the generation of tomographic and related medical diagnostic images of internal bodily organs. This technology offers several advantages over conventional x-ray technology. Firstly, NMR imaging techniques are totally non-invasive and do not generate ionizing radiation. Secondly, NMR imaging produces views of soft tissue structures that are not available with conventional x-rays. Thirdly, NMR imaging methodologies can be repeatedly employed over a period of time to observe metabolic, physiological and chemical changes occurring within the body of the patient. It is therefore seen that NMR imaging technology offers significant advantages in medical diagnostic applications.
However, one of the major requirements for NMR imaging is the presence of a highly uniform magnetic field. This uniform field may be provided by several different means. For example, a permanent magnet may be employed together with magnetic flux concentrating yokes, (or the like). Another method is to employ an electromagnet. A third method for producing a high strength uniform magnetic field is the utilization of superconducting coils disposed in a cryostat. Such superconducting coils have the advantage that, once established, the magnetic field persists for extended periods of time. Accordingly, its energy requirements are relatively small. The correction coil system of the present invention is nonetheless applicable to all of these systems; the permanent magnet, resistive electromagnet and especially the superconductive electromagnet systems.
In a permanent magnet system for supplying the desired uniform field, field uniformity is limited by the materials employed. Likewise, electromagnet designs are limited by manufacturing tolerances. For example, in a volume 20 centimeters by 20 centimeters by 20 centimeters, an error in placement of main superconductive coil windings of as little as 1 mil can mean a deviation in field homogeneity of as much as 100 parts per million. This departure from field homogeneity is generally not acceptable for medical diagnostic imaging purposes since it can produce images with undesirable artifacts.
Accordingly, the need for correction coils exists in all three conventional magnetic field generating techniques. Generally, these correction coils carry a current which is only a small fraction of the main winding current. Furthermore, as a consequence of their shimming purpose, the size and number of turns employed in correction coils is also small relative to the conductors in the main field coils. In general, correction coils produce field gradients in all directions in which the uncorrected field is likely to have gradients. Selective excitation of these coil sets is employed to increase the homogeneity of the field to the desired value.
The usual method of correction coil excitation begins with a measurement of the uncorrected field at many specified field locations. Conventional algorithms are then employed to match the correction coil gradients to those seen in the measured field and determines optimal current settings for all of the correction coils in the set. In general, the correction coil set that is required and its currents are dependent upon the design of the main coil and the homogeneity specifications. Increasing the number of main coils does in fact generally increase the inherent homogeneity of the magnetic field. However, this design method also increases the number of dimensions to which manufacturing and operating tolerances must be applied in order to determine correction coil needs. Furthermore, the presence of ferromagnetic materials in the operating environment of the magnet also requires field correction. To produce the desired axial gradients of the main field, axisymmetric coil sets are required. These coil sets may be even (that is symmetric about the midplane and exhibiting parallel current flow) or odd (that is symmetrically disposed but exhibiting antiparallel currents). Homogeneity specifications are often such that multiple even and odd correction coil sets must be used to provide sufficiently independently adjustable currents to meet them. Problems arise, however, since each even and odd correction coil set, unless properly designed, creates all field gradients allowed by its symmetry. The result of this duplication of gradients is higher currents in all correction coil sets since gradients produced may buck each other in setting the desired field. These bucking currents thus create a demand for higher coil currents or more turns of conductor, both of which are undesirable. It is therefore desirable to eliminate some gradients from each set, however, it is noted that elimination of all gradients but one is a formal impossibility. It is nonetheless advantageous for all coils to consist of one pair and be disposed on a single cylinder at the same radius, for manufacturing efficiency. Accordingly, it is desirable that the gradient elimination employed in the present invention should not require designs which eliminate this possibility. The goal is to minimize total ampere-turns necessary to shim a given field so that increasing the number of coils in a set to eliminate more harmonics is only practical if it reduces the required current accordingly.